Heretofore, it has been common to inject two separate fluids downhole in a well bore hole with one fluid having a radioactive material therein with the other fluid being non-radioactive. A gamma ray detector is lowered within the bore hole and is utilized to determine the location or level of the radioactive fluid. Different pumping rates can be provided for the separate fluids and the interface between the fluids can be determined. Such an injection method has been utilized heretofore to determine the leakage of injected water within a zone. Also, this method may be utilized for the selective injection of acid into the most beneficial zone.
For example, U.S. Pat. No. 2,870,734 dated Jan. 25, 1955 discloses a dual injection method in which two fluids are injected downhole with one of the fluids including a radioactive material. A gamma ray detector is lowered within the bore hole to monitor the interface between the two fluids which changes with different formations and pumping rates. The gamma ray detector or logging instrument is positioned on the end of a conventional electric line for monitoring or determining various characteristics of the bore hole. The logging instrument is received within a work string which receives the treating fluid for discharge from the lower end of the work string. The other fluid which is radioactive flows down the annulus between the work string and casing. In the event the well has been completed for production with a production tubing string within the casing and a packer in the annulus between the production tubing string and casing to block fluid flow, the tubing string and/or the packer must be removed before the injection of the two separate fluids downhole. Upon removal of the production tubing string and associated packers, a work string receiving a logging instrument therein may be inserted.
Other dual injection systems have utilized coiled tubing with a packer in the annulus between the coiled tubing and the casing. In such systems, the packer is positioned generally adjacent the productive zone and the pressure between the zones defined by the packer is measured to determine the injection rate of the two fluids. A sensor is not utilized in determining the interface between the two fluids. The treating pressure above and below the packer is measured for display at a surface location and fluid injection rates are adjusted so that the pressure differential between the fluids is substantially zero.